Depth is perceived when the two eyes of a viewer see slightly different images of a three-dimensional (3-D) scene because they are spaced apart from one another. The brain analyses these two slightly different images and extracts information relating to the depth of the 3-D scene viewed.
The prior art is replete with techniques for producing 3-D images from two-dimensional (2-D) displays. These techniques may be generally divided into two major classes: the auto-stereoscopic class which produces 3-D images which may be viewed freely without wearing spectacles, and the binocular stereoscopic class which produces 3-D images which require the viewer to wear spectacles.
The auto-stereoscopic systems, while being interesting since no spectacles are necessary, suffer a major drawback; the position of the eyes of the viewer must remain in a predetermined position, relative to the display device in order to provide the 3-D effect. An example of such system is described in U.S. Pat. No. 4,953,949 issued on Sep. 4, 1990 to William J. DALLAS.
The binocular stereoscopic systems may be sub-divided in two categories: one using active spectacles, the other using passive spectacles.
Binocular stereoscopic systems using active spectacles operate on a relatively simple principle. Two slightly different images necessary to create a 3-D effect are successively projected by a 2-D display device. The spectacles are synchronized with the display device so that, when the first image is projected, one of the eyes may see it and, when the second image is projected, the other eye may see it. If the repetition rate is sufficiently high, the retinal persistence of the eyes will give the impression that both images are seen at the same time and the brain will interpret these two images as if they were two different views of a 3-D scene as usually viewed by the eyes, thus reconstructing the depth of the 3-D scene.
Binocular stereoscopic systems using passive spectacles have a different principle of operation. One of the images displayed is polarized along a first direction and the other image displayed is polarized along a second direction, usually perpendicular to the first direction. The spectacles comprise one lens having a polarizing filter along the first direction in front of one eye, and another lens having a polarizing filter along the second direction in front of the other eye. Thus, the right eye sees only one of the two projected images and the left eye sees only the other projected image. Again, the brain will interpret these two images as if they were the normal visualisation of a 3-D scene by the eyes.
Many methods for displaying two images on a display device have been developed for binocular stereoscopic systems using passive spectacles.
The two images may be projected by two cathode ray tubes (CRT), filtered with opposite direction polarizing filters and combined through a half-reflective mirror. This technique is expensive and the viewer must be at a specific location to perceive the 3-D effect.
The two images may be projected successively on a CRT display having a LCD shutter installed in front of it. When the first image is displayed, the LCD allows the light, which lies in a first direction, to pass through. When the second image is displayed, the LCD allows the light, which lies in a second direction, to pass through. The second direction is usually perpendicular to the first direction. The passive spectacles have one lens having a polarizing filter along the first direction and the other lens having a polarizing filter along the second direction. Each eye will see a different image and the brain will combine them as previously discussed. The drawback of this technique is the high costs associated with the LCD shutter that must be at least of the same size as the display surface and must have a fast response time between each change of polarization directions.
Another method to display two images on the same screen while using passive spectacles to separate them consist in producing, on a display device, a single composite image formed from the two slightly different images. The method of formation of this composite image may vary. One example of formation of the composite image consists in slicing the two images into narrow strips and forming the composite image by the successive side-by-side concatenation of these narrow strips. Polarizing filters are placed in front of the display surface to polarize the strips coming from the first image along a first direction and to polarize the strips coming from the second image along a second direction, usually perpendicular to the first direction. One problem with this technique is that the resolution of each of the images is, at best, one half of the resolution of the display device, leading to images of lesser quality or to expensive display devices having a higher resolution. An example of such system may be found described in U.S. Pat. No. 5,050,961 issued on Sep. 24, 1991 to Daniel S. VENOLIA.